The present invention relates to a mask for magnetic tape cassettes.
As is known, due to the continuous technical evolution, it is becoming increasingly important to obtain perfect alignment of the windings of magnetic tape of the reel housed in the box-like container which constitutes the cassette.
It is furthermore necessary to have a precise centering of the reel with respect to the middle plane of the cassette, in the direction of tape motion, orthogonal to the axis of rotation of said reels, in order to reduce the azimuth error, that is to say the angular error between the motion of the tape and the gap of the playback head.
To the above one must furthermore add that with the onset of superfast automatic machines for loading magnetic tape, with speeds above 30 m/s, the problem is increasingly felt of having a low torque caused by friction, both during the rapid-winding operations and later in the phase of practical use of the cassette in ordinary players or recorders.
The low friction coefficient reflects positively on the average life of the magnetic tape and the recording head, since the magnetic memory, constituted by the layer of magnetic oxide, has a thickness of a few microns (4/5 .mu.m) which is considerably abrasive, since it is made of metallic oxides.
The ordinary cassette for magnetic tape is constituted by a box-like container, inside which are accommodated, in a freely retained region, two hubs whereon the magnetic tape is fixed.
In order to obtain the above described effects, the solutions currently adopted entail the use of masks, generally constituted by thin sheets of plastic material, such as polyester, which support the reel for its entire extension, specifically in the region affected by the magnetic tape, with constant elastic forces on every undulated profile of the particular shaping of said sheets.
Said masks are currently produced according to two quite distinct methods. A first method entails the provision of raised portions which extend radially with respect to the accommodation holes of the hubs, so as to obtain a distributed support of the reel of magnetic tape.
However, such known types of masks have the severe disadvantage of not being manufacturable in continuous production, since the individual masks must be shaped one by one, by means of molding operations or the like.
Another method which offers the possibility of continuous production has undulated shapes or ridges which generally extend longitudinally with respect to the longitudinal extension of the mask and have the function of supporting the magnetic tape at a plurality of points, so as to obtain both the alignment of the turns and the centering.
Since, as previously mentioned, the magnetic layers of the tape are obtained using highly abrasive metal oxides, low-friction regions must be provided at the regions of contact and are currently produced by applying layers of graphite or of other antifriction material at the tops of the undulations.
This solution, however, has the disadvantage that the provided undulations must support entirely the weight of the reel which, with the tape wound thereon, which can be estimated at approximately 12 g, and which moves alternately from one hub to the other during the motion of the tape inside the cassette.
In order to try and limit this disadvantage, a solution is already known wherein a two-step hub is provided which limits the axial displacement of the hub with respect to the inner walls of the box-like shell of the cassette, so as to obtain improved effects with regard to the centering of the reel of tape in the cassette.
According to the universally acknowledged international standards, the magnetic tape has a width or height of 3.81 mm, and a maximum play of the tape with respect to the inner wall of the box-like shell of 1.30 mm is allowed, while the axial length of the hub must be greater than 3.81 mm, i.e. the height of the tape, and is normally comprised between 3.90 and 4.10 mm.